I always felt that Kingdom Hearts 2's introduction with Roxas was far and away the best part of the whole game as far as plot was concerned. I think it's more of an issue with the way the whole game is paced rather than just the introduction, though. Kingdom Hearts shoves a load of plot at you in the beginning before deviating to Disney world subplots, returns to another original world for a significant burst of plot in the middle, deviates again to Disney world subplots, and then smacks you across the face with the final dose in the end. A slow and dense prologue is just the inevitable consequence of such a structure.